1. Field of the Invention
The invention relates to a continuous process for the preparation of diaryl carbonates from dialkyl carbonates and phenols, using conventional transesterification catalysts, which is characterised in that the starting materials are reacted in a specific, mass-coupled and energy-coupled combination of columns.
2. Description of the Related Art
The preparation of aromatic and aliphatic-aromatic esters of carbonic acid (carbonates) by transesterification, starting from aliphatic esters of carbonic acid and phenols, is known in principle. This is an equilibrium reaction, the position of the equilibrium being almost completely displaced in the direction of the aliphatically substituted carbonates. Therefore, it is relatively easy to prepare aliphatic carbonates from aromatic carbonates and alcohols. However, in order to carry out the reaction in reverse in the direction of aromatic carbonates, it is necessary effectively to displace the highly unfavourably situated equilibrium, where not only do highly active catalysts have to be used, but also an expedient procedure has to be used.
For the transesterification of aliphatic carbonic acid esters with phenols, a multiplicity of effective catalysts have been recommended, such as for example alkali metal hydroxides, Lewis acid catalysts selected from the group comprising the metal halides (DE-OS (German Published Specification) 25 28 412 and DE-OS (German Published Specification) 25 52 907), organotin compounds (EP 879, EP 880, DE-OS (German Published Specification) 34 45 552, EP 338 760), lead compounds (JP-57/176 932), Lewis acid/protonic acid catalysts (DE-OS (German Published Specification) 34 45 553). In the known processes, the transesterification is carried out in a batchwise-operated reactor at atmospheric pressure or at superatmospheric pressure, with or without an additional separation column. In this case, even with the most active catalysts, reactions times of many hours are required until even only moderate conversions of approximately 50% of phenol are achieved. Thus in the batchwise-operated transesterification of phenol with diethyl carbonate at 180.degree. C. using various organotin compounds, as are described in DE-OS (German Published Specification) 34 45 552, yields of diphenyl carbonate in an order of magnitude above 20% are only achieved after a reaction time of approximately 24 hours; in the batchwise-operated transesterification of phenol and dimethyl carbonate with the aid of organotin catalysts as are described in EP-879, the phenol conversion is 34% of the theoretical value after 30 hours.
This means that, because of the unfavourable thermodynamic conditions, the described transesterification reactions in tanks or pressurised autoclaves, even when highly active catalyst systems are used, can only be carried out highly disadvantageously in the sense of an industrial process, since very poor space-time yields and high residence times at high reaction temperatures are obtained, where because of the incomplete transesterification a high distillation effort must additionally be applied which requires further energy.
Such procedures are also particularly disadvantageous since, even using highly selective transesterification catalysts, at the high temperatures and long residence times of many hours, a noticeable-proportion of side-reactions occurs, for example ether formation and elimination of carbon dioxide.
It has therefore been attempted to displace the reaction equilibrium as rapidly as possible in the direction of the desired products by adsorption of the alcohol produced during the transesterification on molecular sieves (DE-OS (German Published Specification) 33 08 921). From the description of this reaction it is evident that a large amount of molecular sieve is required for the adsorption of the reaction alcohol, which greatly exceeds the amount of alcohol being liberated. In addition, the molecular sieves used must be regenerated after just a short time and the rate of conversion to the alkyl aryl carbonate intermediates is relatively low. Even this process, therefore, does not seem to be advantageously usable industrially.
It is known to carry out equilibrium reactions, in particular esterifications and transesterifications, in columns and to displace them in this manner advantageously in the direction of product formation (e.g. U. Block, Chem.-Ing.-Techn. 49, 151 (1977); DE-OS (German Published Specification) 3 809 417; B. Schleper, B. Gutsche, J. Wnuck and L. Jeromin, Chem.-Ing.-Techn. 62, 226 (1990); Ullmanns Encyclopadie der techn. Chemie [Encyclopedia of Industrial Chemistry], 4th edition, volume 3, pp. 375 ff. 1973; ibid. 5th edition, volume B4, pp. 321,1992).
In EP 0 461 274 (WO 91/09832), a continuous transesterification process is described for the preparation of aromatic carbonates in one or more multi-stage columns connected one after the other, dialkyl carbonates or alkyl aryl carbonates, being reacted with phenols and the readily volatile products, that is reaction alcohols and dialkyl carbonates, being withdrawn at the head of the columns and the high-boiling products, that is aryl carbonates, being withdrawn at the foot of the columns.
An already known process principle, carrying out transesterification reactions in columns, is thus being applied here to a specific problem, that is to the transesterification of alkyl carbonates to give aryl carbonates. However, particular engineering measures which permit the transesterification to be carried out more advantageously, matching the apparatuses and procedures to the abovementioned special problems of this difficult transesterification, are not given. Thus, for example, the manner of metering the two starting materials-alkyl carbonate and aromatic hydroxyl compound-is not clearly defined and neither is any advantageous technique emphasised. In a technique according to Diagram 1 of EP 0 461 274, for example, mixtures of these two starting materials are fed into the upper part of the column, the low-boiling reaction products, that is alcohols and unreacted dialkyl carbonate, are withdrawn at the head of the column and the high-boiling reaction products alkyl aryl carbonates and diaryl carbonates are withdrawn, together with unreacted dialkyl carbonates and aromatic hydroxyl compounds, at the foot of the column. In the technique according to Diagram 2 of EP 0 461 274, mixtures of alkyl carbonates and aromatic hydroxyl compounds are supplied at two different points of the column, that is at the upper and lower third of the column, and starting material/product mixtures are withdrawn as in the technique according to Diagram 1 of EP 0 461 274. Neither in the disclosure nor in the examples is a clear differentiation made between conducting the starting materials in co-current and counter-current, although they can have a great influence on the result of the process.
Furthermore, the influence of temperature, pressure, catalyst concentration and liquid residence time is not considered, but only very broad ranges are quoted, even in the restricted claims; for example, temperature ranges from 100.degree. to 280.degree. C., pressure ranges from 0.1 to 200 bar, catalyst concentrations from 0,001 to 50% by weight and liquid residence times from 0.05 to 2 h are quoted.
Different procedures to be preferred in each case for the individual reactions occurring in the conversion of dialkyl carbonates to diaryl carbonates, for example the first transesterification stage from dialkyl carbonates with aromatic hydroxyl compounds to give alkyl aryl carbonates according to Equation 1, the second transesterification stage to give diaryl carbonates according to Equation 2 and the disproportionation according to Equation 3, are not considered in the disclosure. EQU Alk--O--CO--O--Alk+Ar--OH.fwdarw.Alk--O--CO--O--Ar+Alk--OH (Equation 1) EQU Alk--O--CO--O--Ar+Ar--OH.fwdarw.Ar--O--CO--O--Ar+Alk--OH (Equation 2) EQU 2Alk--O--CO--O--Ar.fwdarw.Ar--O--CO--O--Ar+Alk--O--CO--O--Alk (Equation 3)
(Alk=alkyl, Ar=aryl)
The embodiments of this EP 461 274 lead those skilled in the art to the conclusion that, although the transesterification of phenols with dialkyl carbonates can be carried out continuously in a known manner by known processes in columns, it is immaterial by which variant, whether at high or low temperature, in co- or counter-current, at low or high pressure, at large or small molar ratios etc. In brief, one must conclude therefrom that, in the case of this particular transesterification problem, there are no possibilities for improvement and for a more advantageous procedure.
Thus just the examples quoted can be used to evaluate the actual value of this EP.
From these examples it can be seen that in the transesterification of dialkyl carbonates with phenols, even at relatively high temperatures, at elevated pressure and even at molar excesses of dialkyl carbonate of more than 3, only low conversions in the range from 10 to 15% (in the best case approximately 19%) and, especially, only very low space-time yields up to 0.02 kg 1.sup.-1 h.sup.-1 are achieved. This is surprising, especially since very large colons have been used, among them even a 20-plate column 6 m in length and approximately 300 1 in volume. The higher phenol conversion achievable by dialkyl carbonate excesses must in any case be gained, for stoichiometric reasons, at the expense of lower dialkyl carbonate conversions. This means that the dialkyl carbonate withdrawn at the head contains only very low amounts of alcohol and thus, in an industrial process, considerably more unreacted starting product must be circulated and separated off from the small amounts of reaction alcohol. The low space-time yields, for a given production amount per unit time, would make very large reactors and very large distillation capacities necessary.
Although the disproportionation reaction of alkyl aryl carbonates performed in a downstream second column in accordance with Equation 3 does proceed with higher yields, such a disproportionation of alkyl aryl carbonates, in comparison with the further transesterification with phenols, should be seen as less advantageous for an industrial synthesis of diaryl carbonates, since only every second alkyl aryl carbonate molecule is converted into the diaryl carbonate end product and the other half is returned to the starting dialkyl carbonate.
For instance, Examples 22 to 30 of EP 0 461 274, in which reactions are described in two columns connected one after the other and the composition of the head product from the second column is mentioned as feed stream No. 6 in Diagram 4 or 5, it is clear that, in spite of the presence of considerable amounts of phenols, no alcohol is formed in the second reaction stage and accordingly the proportion of the second transesterification stage according to Equation 2 is equal to zero.
In an industrial process for the preparation of diaryl carbonates, specifically of diphenyl carbonate from dimethyl carbonate and phenol, it is not only the phenol conversion which is of importance but also the dimethyl carbonate amount which is necessary to achieve a certain phenol conversion, and the dimethyl carbonate conversion resulting from this. In practice, only low dimethyl carbonate conversions and thus low methanol concentrations in the dimethyl carbonate at the column head will be able to be achieved by such a process variant, for example those of 5 to 10% by weight of methanol. However, in EP 461 274, pure dimethyl carbonate or diethyl carbonate is used without restrictions as starting material. From the viewpoint of the low conversions obtained of dialkyl carbonates of only a few percent, this is understandable and certainly absolutely necessary since, because of the unfavourable equilibrium position, if alcohol-containing dialkyl carbonates were used the conversion rates would be still lower and thus industrially unacceptable. However, methanol forms with dimethyl carbonate an azeotrope of the composition 70% by weight of methanol and 30% by weight of dimethyl carbonate, which can be separated only with great distillation effort.
However, the removal of very small-amounts of the reaction methanol from the dimethyl carbonate product stream requires a particularly high separation effort, as a result of which the return of the unreacted dimethyl carbonate into the transesterification process in pure form can only be achieved with very great effort. This is also of particular economic importance, since, because of the only small dimethyl carbonate conversion rates which can be achieved during a reactor pass, the circulated amounts of dimethyl carbonate are very large.
The aim of an improved transesterification process for the preparation of diaryl carbonates from dialkyl carbonates and phenols would therefore have to be, firstly, to make significant amounts of alcohols tolerable in the dialkyl carbonate starting material stream and, secondly, to promote the transesterification stage according to Equation 2, that is the transesterification of phenol with alkyl aryl carbonate to give diaryl carbonate and to repress the disproportionation of alkyl aryl carbonate.
It can be deduced from the mass action law that even small amounts of alcohols would react with the aryl carbonates already formed, because of the highly unfavourably situated transesterification equilibrium, again in the direction of the starting materials. There therefore seem to be no prospect of realising the above-mentioned first aim. The authors of EP 0 461 274 have apparently also assumed this.
The transesterification of an alkyl aryl carbonate with phenol to give diaryl carbonate according to Equation 2 is, according to the results of EP 461 274, apparently disadvantaged in comparison with the disproportionation of two alkyl aryl carbonate molecules according to Equation 3, or even completely suppressed. It thus appears to be highly questionable whether the second aim can be achieved. For an industrial synthesis, moreover, an increase of the space-time yields above those mentioned in EP 461 274 should be attempted as a third aim in order to decrease the size of the apparatuses. For this as well, EP 461 274 offers no solution.